Method of charging a sealed microwave absorptive gas cell



April 24, 1956 G, w. LEcK ETAL 2,743,048

METHOD OF CHARGING A SEALED MICROWAVE ABSORPTIVE GAS CELL Filed Nov. 29, 1950 M/awmf i550/9pm@ @956:19

EEH- UFF ATTORNEY Unite States Patent NIETHOD F CHARGING A SEALED MICROWAVE ABSORPTIV E GAS CELL George Washington Leck, Princeton Junction, N. J., and William Delmar Hershberger, Los Angeles, Calif., asn siguors toy Radio Corporation of America, a corporation of Delaware Application November 29, 1950, serial No. 198,103

' solaims. (cl. 22e- 204) i pair of gas-tight windows 2, 2. These gas-tight windows 2, 2 are permeable tok microwave transmission. Apparatus for preparing the sealed gas cell 1 comprises a vacuum pump 3` for adjusting-the pressure'of the gas cell to a proper value; a pressurek gauge 5 for yobserving this pressure, a storage reservoir 7 containing gas to be vabsorbed' and adsorbed by the sealed cell walls (hydrogen for example);r another reservoir 9 containing a microwave absorptive resonant gas such as ammonia; a group of valves 11, 13, 15, '17,to control gas flow; and a conduit systemk 19, 2,1, Z3, lconnecting the sealed cell 1l to the rest of 'the system. 'Y Y The sealed gas cell 1 preferably is heated in an oven for a suicient time-to clean the cell of water vapor and varied impurities residing therein. A main control valve 11 and vacuum pump valve 13 are opened and the vacuum pu111np3 removes Ithe undesirable elements from the sealed ce 1.

The vacuum pump control valve 13 then is closed and a valve 15 to the hydrogen storage reservoir 7 is opened. Hydrogen passes from the storage reservoir 7 at a pressure in excess of that at which the sealed cell is to finally operate, through the hydrogen control valve 15, thru the main control valve 11 and into the sealed cell 1. Sufficient hydrogen is introduced into the cell 1 to saturate the walls, then the hydrogen control valve 15 is closed.

absorbed by the porous structure of the cell walls and other such molecules are adsorbed to these walls. Chemical reactions occur between the microwave absorptive molecules and unknown elements existing in the gas cell, these reactions further reducing the number of resonant gas molecules available to absorb microwave energy for frequency control purposes. Loss of these microwave absorbing gas molecules will affect the pressure in' the sealed cell especially at low pressures, since the number of molecules existing in the cell at the operating pressure, which normally is less than l01 mm. of Hg, is much smaller than the number of molecules existing in the cell at atmospheric pressure.

Attempts have been made to reduce pressure variations in the sealed gas cell by coating the walls of the cell with materials having reduced absorption-adsorption characteristics. Loss of electromagnetic energy in these materials isl so great that the microwave energy traveling through the gas cell is too greatly attenuated before this energy had traversed a fraction of the cells length. A materialmost satisfactory for the cell walls is stainless steel. This material conducts electromagnetic energy well and its absorption-adsorption characteristic is reasonably good.

The object of this invention is to provide an improved method for maintenance of a constant partial pressure in a sealed cell.

Another object of this invention is to maintain a gas at a constant partial pressure in a sealed cell when the gas is subject to absorption by or adsorption to the sealed cell walls.

According to the instant invention, the partial pressure of the sealed cell is maintained constant by introducing a gas of a dierent molecular structure into the cell preferably through an aperture, located at a point of maximum voltage in the guide, of cut-off dimensions for the operating frequency. This new gas will be absorbed by and adsorbed to the cell walls, freeing the microwave absorptive gas molecules that would normally be absorbed and adsorbed so that they may absorb microwave energy. The invention will be described in greater detail with the reference to the accompanying drawing wherein the single gure is a schematic diagram of the apparatus used in the preparation of the sealed gas cell.

The sealed gas cell 1 comprises a length of microwave Aconductive waveguide that is sealed off at each end by a After a period of time sutcient for theA walls of the cell to adsorb and absorb the hydrogen molecules, an ammonia gas control valve 17 is opened and ammonia gas, or other desired microwave absorptive gas, flows from the storage reservoir 9 into the sealed cell 1. The ammonia control valve 17 then is closed. The vacuum pump control valve 13 then is opened and the vacuum pump 3 adjusts the presssure in the sealed cell 1 to its nal operating value of the order of 10-1 to 104 mm. of Hg, as observed by a pressure gauge 5. The vacuum pump control valve 13 and the main control valve 11 then are closed. The closing of the main control valve 11 isolates i physically the sealed cell 1 from the rest of the system,

and if desired the cell 1 may be removed at the joint 27 by means of a seal-off tube 29 and incorporated in the fre quency control system.

While ammonia gas has been described as the microwave absorptive resonant gas employed, numerous other such gases may be utilized. Some of these resonant gases are listed in the RCA Review, vol. IX, No. l (March, 1948), at page 40 and include carbonyl sulfide, methyl alcohol, methylamne, and sulfur dioxide. Also, the gases reducing adsorption, absorption, and chemical reaction within the cell may be any of the inert gases. Such gases include helium, neon, and argon and are listed at page 474 of the Handbook of Physics and Chemistry (13th edition, published by the Chemical Rubber Publishing Company in 1928).

We claim as our invention:

l. A method for preparing a sealed gas cell to maing tain a microwave absorptive resonant gas at a .constant partial pressure in'said gas cell comprising evacuating said cell of undesirable elements, introducing a gas into said sealed cell other Athan said absorptive gas and inert with respect thereto, said other gas reducing the effects of absorption and adsorption on said resonant gas, introducing the microwave absorptive resonant gas into said sealed cell, and reducing the pressure of said cell to a final operating value not greater than lO-l millimeters of mercury.

2. A method for preparing a sealed gas cell to maintain a microwave absorptive resonant gas at a constant partial pressure in said gas cell comprising cleaning said sealed cell, evacuating said cell of undesirable elements,

introducing a gas into said sealed cell other than said mamada-pr. 24, 1956:

- 3 ahmtive gas. and inem; with respect thereto, said other gas reducing the effects ofV absorption and adsorption@ said resonant gas, introducing the microwave absorptive resonant gas into said sealec cell, and reducing the pres sure of said cell to. a, final. operating, value nntf `greater' than 1071 rnillinaetersoi mercury.

3f. Amethodiuaccondancewithtclaimflincludingheah ing, said sealed cell., prior tovsaid, evacuation' of` said cell.; 4'. A. method for preparing a` sealed cell' to;- maintaim a microwave. absorptive. resonant gas at a constant. Lraltatl` pressure in= said gas` cell comprising evacuatingsaid; cell,

of undesirable. elements, introducing, a gas, into.' said. cell. which has, a. low molecular weight compared withv the molecular weightof said microwave absorptive gas andis` inert withrespeat thereto, said gasredueing; theietects of' asorption and adsorption on said resonant gas, intraV tain a microwave absorptive resonant gas at a constant partial. pressure. in. said gas cell. comprising evacuatingY Referencesk Cited in the ile of this` patent UNITED STATES PATENTS 1,565,5-79 MacRae Dec. 15, 1 925 1,709,858' Lebrunv g Apr. 23, 1929 1,951,142 Foulke Mar. 13, 1934 2,265,756- Boers'ch Dec. 9.,A 11941 2,284,899 Hedin June 2, 1942 2,427,098 Keizer Sept. 9, 1947 2,449,166 Hershberger Sept. 14, 1948 2,483,768 Hershberger Oct. 4, 1949 2,483,818 Evans Oct. 4, 1949 2,483,829 Hersherger Get. 4,. 1949 

4. A METHOD FOR PREPARING A SEALED CELL TO MAINTAIN A MICROWAVE ABSORPTIVE RESONANT GAS AT A CONSTANT PARTIAL PRESSURE IN SAID GAS CELL COMPRISING EVACUATING SAID CELL OF UNDESIRABLE ELEMENTS, INTRODUCING A GAS INTO SAID CELL WHICH HAS A LOW MOLECULAR WEIGHT COMPARED WITH THE MOLECULAR WEIGHT OF SAID MICROWAVE ABSORPTIVE GAS AND IS INERT WITH RESPECT THERETO, SAID GAS REDUCING THE EFFECTS OF ABSORPTION AND ADSORPTION ON SAID RESONANT GAS, INTRODUCING SAID MICROWAVE ABSORPTIVE RESONANT GAS INTO SAID SEALED CELL, AND REDUCING THE PRESSURE OF SAID CELL TO A FINAL OPERATING VALUE NOT GREATER THAN 10-1 MILLIMETERS OF MERCURY. 